Electromigration-resistant under-bump metallization of nickel-iron alloys for Sn-rich solder bumps in Pb-free flip-clip

ABSTRACT

A process comprises manufacturing an electromigration-resistant under-bump metallization (UBM) flip chip structure comprising a Cu layer by applying to the Cu layer a metallic reaction barrier layer comprising NiFe. The solder employed in the flip chip structure comprise substantially lead-free tin. A structure comprises a product produced by this process. In another embodiment a process comprises manufacturing an electromigration-resistant UBM Sn-rich Pb-free solder bump flip chip structure wherein the electromigration-resistant UBM structure comprises a four-layer structure, or a three-layer structure, wherein the four layer structure is formed by providing 1) an adhesion layer, 2) a Cu seed layer for plating, 3) a reaction barrier layer, and 4) a wettable layer for joining to the solder, and the three-layer structure is formed by providing 1) an adhesion layer, 2) a reaction barrier layer, and 3) a wettable layer. In a further embodiment, the reaction barrier layer comprises metals selected from Ni, Fe, Pd, Pt, Co, Cu and their alloys, and combinations thereof. A structure comprises a product produced by the immediately foregoing process.

FIELD OF THE INVENTION

The field of the invention comprises electromigration-resistantunder-bump metallization (UBM) of nickel-iron alloys for Sn-rich solderbumps employed in Pb-free flip chip applications.

Flip chip or “controlled collapse chip connection,” known by theindustry acronym “C4” comprise the terms for describing the IBM methodof connecting semiconductor and other devices such as integratedcircuits (“IC” chips) and micro electromechanical systems (“MEMS”), bothof which are well known in the art. C4 interconnects employ under bumpmetallization (UBM), sometimes referred to as ball limiting metallurgy(BLM) structures as part of these interconnects.

BACKGROUND OF THE INVENTION AND RELATED ART

To meet the requirements of the EU's RoHS (Reduction of HazardousMaterials) legislation, Pb-free solder technology is being wellimplemented in consumer electronics and mid-range electronic systems,however, several technical issues have been recently identified in thefirst level interconnect structure, such as chip-to-package interactions(CPI), e.g., ILD (Interlayer Dielectric) delamination or cracking (or“white bumps”) in back end of the line structure (BEOL) of the chipduring initial chip joining, and electromigration (EM) in flip-chipjoints under high current density.

The white bump issue is largely due to the inherent high strength ofSn—Ag—Cu (SAC) and SnCu (SC) solders combined with the fragiledielectric materials used in the back-end-of-line (BEOL) structure ofthe advanced complementary metal-oxide-semiconductor (CMOS) chip design.The white bump issue has been successfully addressed by reducing Agcontent in SAC solders combined with an improved design of the BEOLstructure. However, in doing so, the EM resistance of flip-chip solderjoints is greatly compromised in low-Ag, Sn-rich solder bumps. In ourrecent EM studies it has been found that the metallurgical (ormicrostructural) factors are crucial in determining the EM performance,for example, Sn—Ag solders provide a better EM lifetime than Sn—Cusolders. Cf. M. Lu, D-Y Shih, P. Lauro, C. Goldsmith, and D. W.Henderson, “Effect of Sn grain orientation on electromigrationdegradation mechanism in high Sn-based Pb-free solders,” Appl. Phys.Let. v. 92, 211909 (2008).

A fundamental issue owing to the anisotropic properties of Sn singlecrystals is found to be responsible for the premature failures underhigh current EM tests. Lu et al., supra. The solute atoms such as Cu,Ag, or Ni are known to transport much faster along the C-axis than a- orb-axis of the body centered tetragonal Sn single crystal structure.Since a typical flip-chip solder joint consists essentially of only afew Sn crystals, the anisotropic properties of Sn single crystalscritically affect EM performance and other physical/mechanicalproperties when Sn crystals are oriented in a less favorable direction.The high Pb, flip-chip interconnection performs much better in EM teststhan Sn-rich solders, since a Pb crystal is isotropic and Pb has a muchhigher melting point (328° C.) than Sn (232° C.).

In order to control the microstructure of Sn-rich solders and thereby toimprove EM performance and other properties, minor alloying additions toSn-rich solders (such as Ni, Zn, Ti, Sb, Bi, and others) have beenextensively investigated. Among them, Zn addition was found to be themost effective in enhancing EM performance with other beneficialeffects. The microstructure study of Zn-doped solders has revealed thatminor Zn addition stabilizes the microstructure of Sn-rich soldersduring high temperature aging as well as EM tests.

A reaction barrier layer in the UBM structure is important to thereliability of C4 solder joint. A good UBM needs to be wet well by thesolder and form stable, but not too many, intermetallic compounds at theinterface during solder reflow. To meet electromigration reliabilityrequirements the UBM should contain a good reaction barrier layer. Cu isone of the common surface finishes. Cu is wet well by solder, but theinterfacial reaction of Cu and Sn based Pb-free solder is aggressive. Inaddition Cu diffuses rapidly under an electric current. Cu rapidlyconverts into a Cu—Sn intermetallic composition (IMC) under thermal andEM stress, resulting in poor reliability.

Ni UBM is widely used to improve Cu UBM. Although Ni showed slowerinterfacial reaction than Cu, Ni UBM consumption is faster in Pb-freesolder joint, especially when Ni UBM is used as a surface finish forboth the chip and substrates (Cf. FIGS. 2, 3). Various Ni barrier layersover Cu UBM were extensively evaluated for this purpose (S. K. Kang, M.G. Cho, D-Y Shih, S. K. Seo, and H. M. Lee, “Controlling the InterfacialReactions in Pb-free Interconnections by Adding Minor Alloying Elementsto Sn-rich Solders,” Proc. 58^(th) ECTC, Orlando, Fla., May, 2008, pp.478-484, (2008)). Due to aggressive interfacial reactions of Sn-richsolders during multiple reflows and high current EM tests, however, mostNi barrier layers investigated do not provide adequate protection interms of the interfacial reactions and EM resistance.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides structures, articles ofmanufacture and processes that address these needs to not only provideadvantages over the related art, but also to substantially obviate oneor more of the foregoing and other limitations and disadvantages of therelated art such as providing electromigration-resistant under-bumpmetallization of nickel-iron alloys for Sn-rich solder bumps employed inPb-free flip chip applications. Not only do the written description,claims, abstract of the disclosure, and the drawings that follow setforth various features, objectives, and advantages of the invention andhow they may be realized and obtained, but these features, objectives,and advantages will also become apparent by practicing the invention.

To achieve these and other advantages, and in accordance with thepurpose of the invention as embodied and broadly described herein, theinvention comprises a process, composition, and an improved structure ofNi—Fe alloys for Pb-free, solder bumps to be used in high-end serverapplications and addresses the need to enhance electromigrationperformance, i.e., the elimination or reduction of electromigration. Ourprevious work on Ni—Fe UBM for lead-free flip-chip interconnection didnot address the need to enhance electromigration performance. Cf. S. K.Kang, J. Horkans, P. Andricacos, R. Crruthers, J. Cotte, M. Datta, P.Gruber, J. Harper, K. Kwietniak, C. Sambucetti, L. Shi, G. Brouilletteand D. Danovitch, “Pb-Free Solder Alloys for Flip Chip Applications,”Proc. 49th Elec. Comp. Tech. Conf., San Diego, Calif., June 1999, p.283-88; P. Andricacos, M. Datta, H. Deligianni, J. Horkans, S. Kang, K.Kwietniak, G. Mathad, S. Purushothaman, L. Shi, H. Tong, U.S. Pat. No.6,224,690, issued May 1, 2001 “Flip-Chip Interconnections UsingLead-Free Solders.”

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings also set out the invention and areincorporated in and constitute a part of this specification and areincluded to illustrate various embodiments of the invention and togetherwith this written description also serve to explain the principles ofthe invention.

FIG. 1 is a side elevation in cross-section of a prior art UBM structurewith a Ni barrier layer.

FIG. 2 comprises a photomicrograph of Ni UBM consumption afterelectromigration (EM) testing.

FIG. 3 comprises a plot of percentage of resistance increase (ΔR) (%) of8 samples with a Ni/Sn1.8Ag Solder/Ni structure over time after EMtesting where Ni is used as a barrier layer.

FIG. 4 comprises a side elevation in cross-section of a UBM with a NiFealloy as a barrier layer.

FIG. 5, (a) through (f) comprises side elevations in cross-section ofvarious implementations of NiFe as a UBM with either Cu or Au or nothingon top of the NiFe layer.

FIG. 6 comprises a photomicrograph of a UBM with a NiFe and SnAg solderinterface.

FIG. 7 comprises a plot of percentage of resistance increase (ΔR)“Resistivity Change (%)” of 10 samples with a NiFe/Sn1.8AgSolder/NiFeover time after EM testing where Ni/Fe comprises a barrier layer.

DETAILED DESCRIPTION

To achieve these and other advantages, and in accordance with thepurpose of this invention as embodied and broadly described herein, thefollowing detailed embodiments comprise disclosed examples that can beembodied in various forms.

The specific processes, compounds, compositions, and structural detailsset out herein not only comprise a basis for the claims and a basis forteaching one skilled in the art to employ the present invention in anynovel and useful way, but also provide a description of how to make anduse this invention.

The invention comprises a process for manufacturing anelectromigration-resistant under-bump metallization (UBM) flip chipstructure comprising a Cu layer. This comprises applying to the Cu layera metallic reaction barrier layer comprising Ni and Fe, where the UBMstructure comprises a Sn-rich Pb-free solder bump flip chip structure.Pb-free solders for the purpose of the present invention comprisesolders that are substantially free of lead.

In a further embodiment, the present invention comprises boththree-layer and four-layer UBM structures to enhance theelectromigration performance of Pb-free, Sn-rich solder bumps used inhigh-end server applications. A typical UBM structure of the inventioncomprises or consists essentially of 4 layers; 1) adhesion layer, 2) aCu seed layer for plating, 3) a reaction barrier layer, and 4) awettable layer for joining to the solder. The adhesion layer comprisesor consists essentially of metals selected from the group Cr, TiW, Ti,Ta, V, Nb and their alloys, including combinations thereof. In oneembodiment the adhesion layer is form about 200 Å to about 1500 Å thickand in another embodiment the Cu seed layer between said reactionbarrier layer and said adhesion layer comprises a layer from about 5000Å to about 50 um in thickness. In a further embodiment, the Cu seedlayer may be applied as a reaction barrier layer and the adhesion layercomprises Cu, and the thickness of the Cu seed layer is from about 0.01m to about 50 um. In another embodiment, the Cu seed layer is applied asa reaction barrier layer and the adhesion layer comprise Cu to produce afour layer structure, and the four layer structure comprisesTiW/Cu/NiFe/Cu.

The reaction barrier layer comprises or consists essentially of metalsselected from the group Ni, Fe, Pd, Pt, Co, Cu and their alloys,including combinations thereof. The barrier layer is from about 1 um toabout 10 um thick. The wettable layer comprises or consists essentiallyof metals selected from the group comprising Au, Cu, Pd, Pt, Sn Ag andtheir alloys, including combinations thereof. A typical reaction barrierlayer may comprise Ni—Fe alloy compositions comprising about 50Ni toabout 50Fe, about 36Ni to about 64Fe, about 80Ni to about 20Fe, andabout 90Ni to about 10Fe and the like. These amounts comprise the wt %of these metals or elements which is the convention used throughout thisspecification to define these combinations and the other combinations ofelements. In another embodiment, the reaction barrier layer may comprisea range of about 5% to about 70% Fe, or about 10% to about 80% Fe withNi as the balance and alloys thereof. The overall range for the Fe andNi combinations therefore is from about 5% to about 80% Fe with Ni asthe balance and alloys thereof.

Thin films of Ni—Fe alloys can be deposited by the processes ofsputtering, electroplating or electroless plating. To facilitate theplating of UBM, a Cu seed layer may be needed between the reactionbarrier layer and the adhesion layer to form a four-layer structure,such as TiW/Cu/NiFe/Cu. The barrier layer comprising Ni and Fe is fromabout 1 um to about 10 um thick.

To enhance the EM performance the thickness of the Cu seed layer can beincreased to about 5 um to about to about 50 um. In another embodimentof the invention, the top Cu layer on both the three-layer or four-layerstructures may be included or omitted, depending on the manufacturingrequirement. In addition, Ni—Fe can be used as a barrier layer with orwithout a wetting layer on the Cu pillar or thick Cu pedestal structureto further improve EM reliability, wherein the Ni—Fe plated pillar orpedestal is an alternative embodiment on either or both sides of thesolder joint, i.e., on either chip or substrate or on both. Thesubstrates comprise art-known materials such as polymeric materialsincluding phenolics, epoxies, and polyimides and equivalents known inthe art, or ceramics such as alumina and equivalents known in the art.

Ni—Fe metallurgy can be used as barrier layer on bumps either as a topcapping layer or conformably deposited over the exposed surface of pad,pillar or pedestal structures to further improve EM performance and CPIcompliance. In an extended application, the NiFe alloying layer can beplated and manufactured by other art-known means on the substrates padto enhance EM performance. The UBM layer or pad is from about 0.5 um toabout 5 um thick, the pedestal is from about 5 um to about 30 um thick,and the pillar is from about 30 um to about 50 um thick. A pre-tin layeror an alloy composed of at least one of Au, Cu, Pd, Ag, In, and Sn,including combinations thereof can be deposited on the NiFe beforesoldering.

As noted, the invention also comprises three-layer UBM structures forenhanced electromigration resistance of Pb-free, Sn-rich solder bumpsused in high-end server applications. The new three-layer UBM structurecomprises or consists essentially of 3 layers; 1) adhesion layer, 2)reaction barrier layer, and 3) wettable layer. The adhesion layercomprises or consists essentially of metals selected from Cr, TiW, Ti,Ta and alloys thereof, including mixtures thereof. The reaction barrierlayer adhesion layer comprises Cu, and the thickness of the Cu seedlayer is from about 0.01 μm to about 50 um. In another embodiment, theCu seed layer is applied as a reaction barrier layer and the adhesionlayer comprise Cu to produce a four layer structure, and the four layerstructure comprises TiW/Cu/NiFe/Cu.

The reaction barrier layer comprises or consists essentially of metalsselected from the group Ni, Fe, Pd, Pt, Co, Cu and their alloys,including combinations thereof. The barrier layer is from about 1 um toabout 10 um thick. The wettable layer comprises or consists essentiallyof metals selected from the group comprising Au, Cu, Pd, Pt, Sn Ag andtheir alloys, including combinations thereof. A typical reaction barrierlayer may comprise Ni—Fe alloy compositions comprising about 50Ni toabout 50Fe, about 36Ni to about 64Fe, about 80Ni to about 20Fe, andabout 90Ni to about 10Fe and the like. These amounts comprise the wt %of these metals or elements which is the convention used throughout thisspecification to define these combinations and the other combinations ofelements. In another embodiment, the reaction barrier layer may comprisea range of about 5% to about 70% Fe, or about 10% to about 80% Fe withNi as the balance and alloys thereof. The overall range for the Fe andNi combinations therefore is from about 5% to about 80% Fe with Ni asthe balance and alloys thereof.

Thin films of Ni—Fe alloys can be deposited by the processes ofsputtering electroplating or electroless plating. To facilitate theplating of UBM, a Cu seed layer may be needed between the reactionbarrier layer and the adhesion layer to form a four layer structure,such as TiW/Cu/NiFe/Cu. The barrier layer comprising Ni and Fe is fromabout 1 um to about 10 um thick.

To enhance the EM performance the thickness of the Cu seed layer can beincreased to about 5 um to about to about 50 um. In another embodimentof the one embodiment the Ni—Fe or Cu pedestal is from about 5 um toabout 30 um thick, and in another embodiment the Ni—Fe or Cu pillar isfrom about 5 um to about 50 um thick.

In accordance with one embodiment of the present invention, a TiWadhesion layer is sputtered at a thickness of about 500 Å to about 1500Å or about 1000 Å on a substrate as described herein. The thickness ofthe adhesion layer can vary widely as long as both good adhesion andgood barrier properties are maintained, since the blanket TiW must beetched as the final step in forming the BLM, the film thickness shouldbe minimized consistent with adequate performance. An alternativeadhesion layer comprises sputtered Cr at a thickness of about 500 Å toabout 1500 Å or about 1000 Å.

Referring to the Figures; in FIG. 1, 100 is a prior art semiconductordevice including 110, a plated or sputtered Ni UBM, 102 a semiconductorchip with functional circuitry, 104 the last metal layer of the BEOLstructure with wiring channels. Element 106 is a photosensitivepolyimide that defines vias in the opening of the UBM, 108 is a stack ofmetal films of adhesion and seed layers for UBM, 112 is a solder, 114 anunderfill, 116 Cu wiring on substrate 118, and 120 a substrate whichconsists of an organic, ceramic, or silicon interposer.

In FIG. 2, 200 is a photomicrograph cross-section image of a prior artsolder bump after EM stress in which 210 is a semiconductor chip withfunctional circuitry, 212 is a void formed in solder after EM testing,216 is a solder, 218 is an underfill, 219 a metal finish on substrate inthis case is Ni(P), and 220 is a substrate. 214 shows a Ni UBM consumedcompletely after EM testing and illustrate how Ni by itself does notprovide a robust EM barrier. The EM test is performed at 150 C with 500mA current. The electron flow direction is from silicon chip down to thesubstrate.

FIG. 4 comprises a side elevation in cross-section of a UBM with a NiFealloy as a barrier layer, wherein 400 comprises an illustration of a UBMwith NiFe as a barrier layer, further comprising elements 402, asemiconductor chip with functional circuitry, 404 a last metal BEOLstructure with wiring channels, 406 a photosensitive polyimide, 408 astack of metal films of adhesion and seed layer for UBM, 410 a plated orsputtered NiFe UBM, 412 a solder, 414 an underfill, 416 a Cu receivingpad on a substrate, 418 Cu wiring in a substrate, and 420 a substratewhich comprises either an organic, ceramic, or silicon interposer.

One embodiment as depicted in FIGS. 4 and 5( a) consists of atraditional UBM structure with NiFe barrier layer, TiW/NiFe/Cu. Cu iscompletely dissolved in solder after reflow and is not depicted in thefigure.

FIG. 5 comprises various implementations of NiFe as a UBM comprising 510a microcircuit chip device or a silicon interposor, 512 a NiFe UBM,either thick (d) or thin (a), stack which comprises an adhesion layer,seed layer for plating (no need for sputter NiFe film), and optionalwetting layer, 514 a Cu layer, 518 a NiFe barrier layer surrounding a Culayer, 520 a ceramic or organic substrate and, 530 an interposer withinterconnects on both sides.

FIG. 5, (a) through (f) illustrate various implementations of NiFe as aUBM with either Cu or Au or nothing on top of the NiFe layer. A thinadhesion layer, such as Ti and TiW, is first deposited on thepassivation layer and via openings on the chip followed by a thin Cuseed layer deposition. NiFe is subsequently deposited on the sputteredCu layer. Several structural variations (a) through (e) are describedbelow:

-   (a) NiFe as thin UBM (about 1 um to about 5 um) on a semiconductor    chip or interposor;-   (b) NiFe on top of thick Cu UBM (about 1 um to about 50 um) and    comprises the embodiment of NiFe which serves as a barrier layer for    thick Cu UBM to reduce current crowding and thereby improves EM    reliability;-   (c) Electroless plated NiFe covers thick Cu UBM (about 5 um to about    50 um) and comprises a variation of the embodiment of FIG. 5( b)    wherein electroless plated NiFe covers the exposed surfaces of Cu    UBM to prevent undercutting due to Cu/Sn interaction during reflow;-   (d) Thick (about 1 um to about 50 um) NiFe UBM and comprises an    embodiment that employs thick NiFe UBM;-   (e) NiFe as a substrate surface finish, either laminate or HPGC    substrates and comprises an embodiment that employs NiFe for organic    or ceramic substrate surface finishes.-   (f) NiFe as a substrate surface finish for an interposer with    interconnects on both sides.

FIG. 6 comprises a photomicrograph 600 of a UBM with a NiFe and SnAgsolder, further comprising 610, a SnAg solder, intermetallic layer 612 aNiFe and solder interface with Fe₂Sn intermetallic compound formed aftersolder reflow, i.e., an interface structure of TiW/NiFe/Cu and Sn1.8Agsolder after reflow, 614 a NiFe UBM layer with sputtered Ni50Fe50 alloy,616 a TiW adhesion layer, and 618 a silicon substrate. The intermetalliclayer 612, an IMC layer, comprises a thin, dense, uniform Fe₂Sn layerformed at the interface, and is stable even after prolonged thermal andelectromigration stress, and protects the UBM from EM damage.

FIG. 7 comprises the plot of resistance change of the NiFe/SnAgsolder/NiFe structure. Compared to FIG. 3, the Ni/Solder/Ni case, earlyEM failures are eliminated, and the EM reliability is improved with NiFeUBM. The NiFe is a sputtered film with composition of Ni50Fe50 and thethickness is about 6 um. A 1500 Å TiW adhesion layer is in between theNiFe film and chip. The solder composition is Sn1.8Ag. The EM testing isconduced at 150° C. and 500 mA.

The sample preparing and testing conditions are identical for all of theten samples. The only difference between the samples in FIG. 3 and FIG.7 is the UBM composition, where Ni is the UBM composition in FIG. 3 andNiFe is the UBM composition in FIG. 7.

In other embodiments the NiFe composition comprises about 5% to about70% Fe with the balance Ni; we can employ an optional wetting or cappinglayer comprising Au or Cu; and the NiFe thickness can vary from about0.1 um to about 50 um or from about 1 um to about 10 um.

Without being bound by any theory it would appear that during theapplication of the layers and/or soldering process the metals in thevarious layers intermix to form alloys and/or mixtures different thanthose originally applied to the various surfaces, so the exactcomposition of the layers after deposition or after formation of thesolder joints can vary from the original compositions, i.e., thecompositions as they existed prior to their deposition as layers orprior to soldering. Accordingly, we claim the invention as a product orarticle of manufacture produced by the process of the invention.

Throughout this specification, abstract of the disclosure, and in thedrawings, the inventors have set out equivalents, including withoutlimitation, equivalent elements, materials, compounds, compositions,conditions, processes, structures and the like, and even though set outindividually, also include combinations of these equivalents such as thetwo component, three component, or four component combinations, or moreas well as combinations of such equivalent elements, materials,compositions conditions, processes, structures and the like in anyratios or in any manner.

Additionally, the various numerical ranges describing the invention asset forth throughout the specification also includes any combination ofthe lower ends of the ranges with the higher ends of the ranges, and anysingle numerical value, or any single numerical value that will reducethe scope of the lower limits of the range or the scope of the higherlimits of the range, and also includes ranges falling within any ofthese ranges.

The terms “about,” “substantial,” or “substantially” in any claim or asapplied to any parameters herein, such as a numerical value, includingvalues used to describe numerical ranges, means slight variations in theparameter. In another embodiment, the terms “about,” “substantial,” or“substantially,” when employed to define numerical parameter include,e.g., a variation up to five per-cent, ten per-cent, or per-cent, orsomewhat higher or lower than the upper limit of five per-cent, tenper-cent, or 15 per-cent. The term “up to” that defines numericalparameters means a lower limit comprising zero or a miniscule number,e.g., 0.001. The terms “about,” “substantial” and “substantially” alsomean that which is largely or for the most part or entirely specified.The inventors also employ the terms “substantial,” “substantially,” and“about” in the same way as a person with ordinary skill in the art wouldunderstand them or employ them. The phrase “at least” means one or acombination of the elements, materials, compounds, or conditions, andthe like specified herein, wherein “combination” is defined above. Theterms “written description,” “specification,” “claims,” “drawings,” and“abstract” as used herein refer to the written description,specification, claims, drawings, and abstract of the disclosure asoriginally filed, or the written description, specification, claims,drawings, and abstract of the disclosure as subsequently amended, as thecase may be.

All scientific journal articles and other articles, including internetsites, as well as issued and pending patents that this writtendescription mentions including the references cited in such scientificjournal articles and other articles, including Internet sites, and suchpatents, are incorporated herein by reference in their entirety and forthe purpose cited in this written description and for all otherdisclosures contained in such scientific journal articles and otherarticles, including internet sites as well as patents and the aforesaidreferences cited therein, as all or any one may bear on or apply inwhole or in part, not only to the foregoing written description, butalso the following claims, abstract of the disclosure, and appendeddrawings.

Applicants reserve the right to amend this specification and thedrawings filed herein without prejudice or disclaimer and in doing so,they do not disclaim, abandon, or forfeit any invention directly orindirectly represented thereby, and hereby rescind any implieddisclaimer that may be inferred from applicants' amendments orcancellation in the course of prosecution of this or any parent,continuation, continuing, continuation in part, divisional, terminallydisclaimed, or foreign patent application (related patent application orapplications), and specifically preserve all and any inventions embodiednot only by the original claims but also the written description,abstract of the disclosure, and drawings of this application and anyrelated patent application or applications, and preserve the right toprosecute claims to these inventions in any related patent applicationor applications filed prior to or subsequent to the filing of thisapplication.

Although the inventors have described their invention by reference tosome embodiments, other embodiments defined by the doctrine ofequivalents are intended to be included as falling within the broadscope and spirit of the foregoing written description, and the followingclaims, abstract of the disclosure, and appended drawings.

We claim:
 1. A process for manufacturing an electromigration-resistantUBM Sn-rich Pb-free solder bump flip chip structure wherein saidelectromigration-resistant UBM structure consists essentially of afour-layer electromigration-resistant UBM structure or a three-layerelectromigration-resistant UBM structure, and wherein said four layerstructure consists essentially of, 1) an adhesion layer, 2) a Cu seedlayer for plating, 3) a reaction barrier layer, and 4) a wettable layerfor joining to said solder, and said three-layer structure is formed byapplying to said structure, 1) an adhesion layer, 2) a reaction barrierlayer, and 3) a wettable layer for joining to said solder, wherein said4 layer UBM structure consists essentially of a Cu layer and saidreaction barrier layer consists essentially of a Ni—Fe alloy or Cuapplied to said Cu layer to eliminate or reduce electromigration.
 2. Theprocess of claim 1 wherein said adhesion layer consists essentially ofmetals selected from Cr, TiW, Ti, Ta, V, Nb and their alloys, andcombinations thereof.
 3. The process of claim 1 wherein said adhesionlayer consists essentially of TiW or Cr.
 4. The process of claim 3wherein said adhesion layer is form about 200 Å to about 1500 Å thick.5. A process for manufacturing an electromigration-resistant UBM Sn-richPb-free solder bump flip chip structure wherein saidelectromigration-resistant structure consists essentially of a fourlayer electromigration-resistant UBM structure or a three-layerelectromigration-resistant UBM structure, and wherein said four layerstructure consists essentially of, 1) an adhesion layer, 2) a Cu seedlayer for plating, 3) a reaction barrier layer, and 4) a wettable layerfor joining to said solder, and said three-layer structure is formed byapplying to said structure, 1) an adhesion layer, 2) a reaction barrierlayer, and 3) a wettable layer for joining to said solder, wherein saidUBM 4 layer structure consists essentially of a Cu layer and saidreaction barrier layer essentially of a Ni—Fe alloy or Cu applied tosaid Cu layer to eliminate or reduce electromigration, wherein saidreaction barrier layer is selected from xNi-yFe, Pt, Cr or Co, and theiralloys, and combinations thereof.
 6. The process of claim 5 wherein saidreaction barrier layer consists essentially of thin films of Ni—Fedeposited by the processes consisting essentially of sputtering,electroplating or electroless plating.
 7. The process of claim 5 whereinsaid reaction barrier layer consists essentially of films of Ni—Feranging in thickness of from about 1 um to about 50 um.
 8. A productmade by the process of claim
 1. 9. The process of claim 1 furtherconsisting essentially of a reflow of said Sn-rich Pb-free solder bumpto dissolve said Cu layer in said solder bump where said Cu layer abutssaid solder bump.
 10. A process for manufacturing anelectromigration-resistant UBM Sn-rich Pb-free solder bump flip chipstructure wherein said electromigration-resistant structure consistsessentially of a four layer electromigration-resistant UBM structure ora three-layer electromigration-resistant UBM structure, and wherein saidfour layer structure consists essentially of, 1) an adhesion layer, 2) aCu seed layer for plating, 3) a reaction barrier layer, and 4) awettable layer for joining to said solder, and said three-layerstructure is formed by applying to said structure, 1) an adhesion layer,2) a reaction barrier layer, and 3) a wettable layer for joining to saidsolder, wherein said UBM 4 layer structure consists essentially of a Culayer and said reaction barrier layer essentially of a Ni—Fe alloy or Cuapplied to said Cu layer to eliminate or reduce electromigration,wherein said reaction barrier layer is selected from xNi-yFe, CrCu, orNiCu.
 11. A product made by the process of claim
 10. 12. The process ofclaim 1 wherein said reaction barrier layer consists essentially ofabout 5% to about 80% Fe with Ni as the balance and consists essentiallyof alloys thereof and mixtures thereof.
 13. The process of claim 12wherein said UBM structure consists essentially of a substrate and acircuit board and incorporates a Cu pad, Cu pedestal, or Cu pillarstructure between said substrate and said circuit board.
 14. The processof claim 13 wherein said Ni—Fe alloy is positioned as a barrier layerwith or without said wetting layer on said Cu pad, Cu pillar, or Cupedestal structure to further improve EM performance and CPI compliance.15. The process of claim 12 wherein said UBM structure consistsessentially of a chip and a substrate and incorporates a pad, pedestal,or pillar structure between said substrate and said chip consistingessentially of said UBM structure and said Ni—Fe alloy is operativelyassociated with said pad, said pillar, or said pedestal.
 16. The processof claim 12 wherein said Ni—Fe alloy operatively associated with saidpad, said pillar, or said pedestal is deposited on either said chip, orsaid substrate, or both.
 17. The process of claim 15 wherein said Ni—Fealloy is applied as a barrier layer on said solder bumps either as a topcapping layer or conformably deposited over the exposed surface of saidpad, said pillar or said pedestal structure to further improve EMperformance and CPI compliance.
 18. The process of claim 12 wherein saidwettable layer consisting essentially of at least one of Au, Cu, Pd, Pt,Ag, In and Sn and alloys thereof or combinations thereof is deposited onsaid Ni—Fe alloy prior to soldering said structure.
 19. The process ofclaim 12 consisting essentially of depositing said wettable layer of atleast one of Au, Cu, Pd, Pt , Ag, In and Sn, and their alloys andcombinations thereof on said Ni—Fe alloy and subsequently soldering saidstructure.
 20. The process of claim 1 wherein said reaction barrierlayer consists essentially of Ni—Fe compositions consisting essentiallyof about 50 Ni to about 50 Fe, about 36 Ni to about 64 Fe, about 80 Nito about 20 Fe, and about 90 Ni to about 10 Fe.
 21. The process of claim1 wherein said wettable layer consists essentially of metals selectedfrom Au, Cu, Pd, Pt, Sn, In, and Ag, and their alloys, and combinationsthereof.
 22. The process of claim 1 wherein said Cu seed layer isapplied as a reaction barrier layer and said adhesion layer consistsessentially of Cu.
 23. The process of claim 22 wherein said adhesionlayer comprises consists essentially of Cu, and the thickness of said Cuseed layer is from about 0.01 um to about 50 um.
 24. The process ofclaim 22 wherein said Cu seed layer is applied to said NiFe alloy as areaction barrier layer and said adhesion layer comprise Cu to produce afour layer structure, and said four layer structure consists essentiallyof TiW/Cu/NiFe/Cu.
 25. The process of claim 1 wherein said four layerstructure consists essentially of TiW/Cu/NiFe/Cu.
 26. The process ofclaim 1 wherein said Cu seed layer between said reaction barrier layerand said adhesion layer comprises consists essentially of a layer fromabout 0.01 um to about 50 um in thickness.
 27. The process of claim 1wherein a top Cu wetting layer on both the three- or four-layerstructures may be included or omitted.
 28. The process of claim 1wherein said UBM structure incorporates a pad, pedestal, or pillarstructure between a substrate and a circuit board consisting essentiallyof said UBM structure.
 29. The process of claim 1 wherein said UBMstructure comprise a substrate and a circuit board and incorporates a Cupad, Cu pedestal, or Cu pillar structure between said substrate and saidcircuit board.
 30. A product made by the process of claim
 5. 31. Theprocess of claim 1 further consisting essentially of a reflow of saidSn-rich Pb-free solder bump to dissolve said Cu layer in said solderbump where said Cu layer abuts said solder bump.